Melt migration under ocean ridges: inferences from reactive transport modelling of dunite bodies

Tabular dunite bodies within the ophiolitic upper mantle are widely considered to locate pathways for melt transport under ocean ridges. They probably formed by in-situ transformation of mantle harzburgite to dunite along melt channels. In order for the melts to be reactive with the host mantle they cannot be locally derived but must come from greater depth without significant re-equilibration. We have studied mantle hosted dunites in the Bay of Islands Ophiolite (BOIO), expecting that the dunites contain information about the nature of the melts feeding the spreading center (cf. Kelemen et al. 1995). Analysis of the rare clinopyroxene (cpx) grains of the dunites reveals a refractory trace element composition even though the average crustal signature in the BOIO is MORB-like in terms of the rare earth elements (REE). The analysed cpx grains are somewhat enriched with respect to the host harzburgite but dramatically depleted with respect to MORB (Fig. 1). In the context ofreplacive dunite formation the question is whether during the reaction of harzburgite to dunite triggered by a relatively enriched melt (MORB in this case), the refractory trace element nature of the original harzburgite may be partially preserved. A numerical scheme was developed to simulate melt-host reaction triggered by diffusive and advective transport of melt within a porous network. A melt channel carrying reactive melt acts as boundary condition. Exchange between melt and host occurs by diffusion and via solution-precipitation. The reaction proceeds with a finite, but fast reaction rate simulating the nearly grain scale transition observed between harzburgite and dunite. The reaction continues until a saturation concentration of silica is reached. With the chosen reactivity of the melt (1 unit melt converts 2.2 units of harzburgite to dunite) the velocity of the reaction front in an